Projection optical systems present many optical design challenges, several of which require a tradeoff between conflicting design goals. Foremost is the requirement of producing a high-quality image, characterized by low distortion, high resolution, high contrast ratio, high light throughput, and uniform screen illumination. In addition to the image quality requirement, the operational environment for many optical projectors introduces further constraints. For example, many projectors must project images large enough to be seen by many people, yet operate in a rather small conference room. This constraint requires the projection system to have a relatively wide field angle so that a large image may be produced in spite of a short projection distance.
Additionally, projection systems are rarely positioned level with the center of the projection screen. Because the projector system and the center of the screen are not located at the same elevation, the image may exhibit a keystoning effect. Keystoning occurs when an image is projected at an angle onto a screen, such as when a table-top overhead projector projects an image upward onto a wall screen. Due to the projection angle, the side of the image furthest from the projector, typically the top, is wider than the side of the image closest to the projector--causing the image to resemble the wedge-shaped keystone at the crown of a stone archway. Wide field angles exacerbate the problem of keystoning.
Furthermore, some application require a zoom lens to allow the projector to adapt to various operating environments. Projection zoom lenses are difficult to combine with reflective modulators since they must allow adequate room behind the lens to separate the incident and reflected light beams. The incident and reflected beams are nearly coincident when a reflective modulator is used. When the incident and reflected beams are so close, it is very difficult to prevent the projection optics from interfering with the illumination optics while limiting the overall size of the projector. Vignetting occurs when the illumination path is partially blocked by the projection optics.
Existing zoom lenses that have wide field angles, low distortion, high resolution, high contrast ratio, high light throughput, and uniform screen illumination are either too small format, have too short of a back focal distance to be useful for DMD based projection systems, or require too much space for the zoom mechanism near the spatial light modulator and illumination optics.
Therefore, there is a need in the art for a high resolution projection zoom lens having a long back focal distance, a wide field angle, high contrast ratio, low distortion, high light throughput, and uniform screen illumination that may be used with a reflective spatial light modulator without interfering with the illumination optics.